We use numerical simulations to show that a suitably dimensioned periodic arrangement of vertical metallic metal-dielectric-metal nanocavities supports a hybrid plasmonic mode whose spatial electric field distribution is suitable for use in infrared photodetectors based on an unpatterned semiconductor thin-film absorbing layer. The partially localized nature of the hybrid mode offers reduced sensitivity to the angle of incoming light and smaller pixel sizes compared with surface plasmonic modes coupled by diffraction.